ABSTRACT
I. INTRODUCTION In the last 20 years or so, the development of metal matrix composites (MMCs) has been one of the major innovations in materials. The early work on composites concentrated on continuous reinforcements such as graphite and boron fibers, but the prohibitive cost of continuous fibers with high mechanical properties restricted their use to aerospace applications. In addition, composites reinforced with continuous fibers have no secondary fabrication capability, because the fibers are heavily damaged by any forming operation such as extrusion, rolling, or forging. This means that continuously reinforced composites must be used in essentially the same form in which they are initially fabricated, restricting them to applications involving simple shapes such as sheet and tube. While development work is continuing on continuously reinforced MMCs [1], particularly for Ti/SiC composites, for which several processing routes have been developed [2-4], more recent efforts have concentrated on discontinuous reinforcement. The availability of SiC whiskers in the mid-1960s led to considerable work on whisker reinforced composites, and these composites have been the subject of several reviews [5,6]. However, the high cost of whiskers together with toxicology concerns have severely restricted commercialization in MMCs, although they remain of interest for ceramic matrices. A significantly cheaper reinforcement became available with the development of short staple A1203 and mullite fiber [7,8], and these were commercialized in Al matrices for selective reinforcement of engine parts. An example of such an application is the reinforcement of the ring land area of diesel engine pistons to provide wear resistance [9]. These composites are made by pressure infiltration of a preform, which can be achieved during squeeze casting
to form the part itself, so it is a relatively low cost process. However, there is still no secondary fabrication capability, and the performance of the composite is critically dependent on the quality of the preform, which itself depends on a wide range of factors such as binder content and shot content (shot is nonfiber material produced during initial fiber processing).
